The Rus', also known as Russes, were a people in early medieval Eastern Europe. The scholarly consensus holds that they were originally Norsemen, mainly originating from present-day Sweden, who settled and ruled along the river-routes between the Baltic and the Black Seas from around the 8th to 11th centuries AD. In the 9th century, they formed the state of Kievan Rusʹ, where the ruling Norsemen along with local Finnic tribes gradually assimilated into the East Slavic population, with Old East Slavic becoming the common spoken language. Old Norse remained familiar to the elite until their complete assimilation by the second half of the 11th century, and in rural areas, vestiges of Norse culture persisted as late as the 14th and early 15th centuries, particularly in the north.

The Norsemen were a North Germanic linguistic group of the Early Middle Ages, during which they spoke the Old Norse language. The language belongs to the North Germanic branch of the Indo-European languages and is the predecessor of the modern Germanic languages of Scandinavia. During the late eighth century, Scandinavians embarked on a large-scale expansion in all directions, giving rise to the Viking Age. In English-language scholarship since the 19th century, Norse seafaring traders, settlers and warriors have commonly been referred to as Vikings. Historians of Anglo-Saxon England distinguish between Norse Vikings (Norsemen) from Norway, who mainly invaded and occupied the islands north and north-west of Britain, as well as Ireland and western Britain, and Danish Vikings, who principally invaded and occupied eastern Britain.